1. Field of the Invention
The present invention relates to a semiconductor amplifying apparatus and a communication terminal apparatus which can be applied to, for example, portable telephones of a quasi-micro wave band and semiconductor amplifying apparatuses for amplifying the power of the quasi-micro waves.
2. Description of the Related Art
At present, the operation of land mobile communication systems, represented by the PHS (Personal Handphone System) and a transmission scheme, referred to as a "PDC" (Personal Digital Cellular) scheme, using a quasi-micro wave band (0.8 to 2 GHz), has been inaugurated in automobile telephone and portable telephone services in order to support an increasing number of subscribers. Since communication terminal apparatuses for use in such a mobile communication systems particularly attach importance to their portability, they are essentially small and battery-driven. Particularly, power amplifying apparatuses for transmission require a large amount of current consumption or power consumption, a reduction in current consumption is an important problem.
The above-mentioned systems such as PHS, PDC, and so on, adopt modulation schemes which are modifications to the quadrature phase shift keying (QPSK) modulation such as .pi./4 shift differential quadrature phase shift keying (QPSK). Power amplifying apparatuses for use in these mobile communication systems for transmission are required to have linear or quasi-linear operation characteristics.
For reducing the generation of distortion as much as possible in power amplifying apparatuses, an operation point of an A-grade or an AB-grade near an A-grade may be selected. The selection of this operation point causes a power addition efficiency to decrease due to a large amount of current consumption.
On the other hand, there is an amplifier which sets its operation point at an AB-grade near a B-grade to generate a predetermined output and corrects distortion generated in this event by a special means. A predistortion amplifier or the like may be taken as an example. However, since this type of amplifier has a complicated configuration, implementation of the amplifier in a one-chip semiconductor amplifier device is practically difficult. Distortion correction techniques as mentioned above, in turn, are extremely effective in realizing a battery-driven highly efficient linear amplifier, extensive research and development on such techniques are still currently in progress.
A technique for correcting distortion is described, for example, in an article entitled "Analysis on Phase Characteristics of GaAs FET Power Amplifier for Digital Portable Telephone", Transactions of the Institute of Electronics, Information, and Communication Engineers C-1, VOL. J76-C-1, No.11, pp.414-421 (November 1993). This article shows that a phase deviation of a fundamental wave in output power with respect to increased input power varies depending on an impedance of a matching circuit connected to an input terminal or an output terminal of a field effect transistor (hereinafter referred to as an "FET" (Field Effect Transistor)). This article also shows that distortion can be reduced at the same output level by making the phase deviation smaller.
An example of an actual design for a semiconductor amplifying apparatus employing similar techniques is described in an article entitled "Design of Linear Power Amplifiers for Digital Cordless", 1994 Spring National Meeting of the Institute of Electronics, Information, and Communication Engineers, C-106. According to this design example described in the article, a modulated signal of a PHS system can be outputted with distortion reduced to as low as 22.7 dBm by reducing a phase deviation of a fundamental wave.
It should be noted that a semiconductor amplifying apparatus is only practically required to have adjacent channel power (hereinafter referred to as "ACP") satisfying a standard value of -55 dBc or less irrespective of the amount of phase deviation of a fundamental wave. Thus, the impedance of a matching circuit connected to a drain terminal of a GaAs junction FET (hereinafter referred to "GaAs JFET") is fixed, while the impedance of a matching circuit connected to a gate terminal of the same is varied to apply input power to the gate terminal. Under these conditions, measurements are made to the output power which provides the full standard value (hereinafter referred to as the "effective output") and a phase deviation amount of a fundamental wave.
As illustrated in a Smith chart of FIG. 30, the measurements result in characteristic curves 1, 2, 3 and 4 (represented by solid lines in the figure) for the effective outputs P.sub.0 and characteristic curves 5, 6 and 7 (represented by broken lines in the figure) for phase deviation amounts of the phase .phi. of the fundamental wave on a Smith chart.
The characteristic curves 1, 2, 3 and 4 represent P.sub.0 =23.3 dBm, P.sub.0 =23.4 dBm, P.sub.0 =23.5 dBm, and P.sub.0 =24.0 dBm, respectively, while the characteristic curves 5, 6 and 7 represent .phi.=5.degree., .phi.=4.degree., and .phi.=3.degree. respectively.
A current flowing into the GaAs JFET, which is determined by voltages applied to the drain terminal and the gate terminal, remains at 120 mA irrespective of variations in impedances of the matching circuits connected to the respective terminals. The relationship between the phase deviation of the fundamental wave and the effective output can be understood from FIG. 30.
However, FIG. 30 reveals that no causal relation is established between the phase deviation amount of the fundamental wave and the effective output. More specifically, a central position of distribution of contour-like characteristic curves 1 to 4 for the effective output P.sub.0 is far spaced away from a central position of distribution of contour-like characteristic curves 5 to 7 for the phase deviation amounts of the phase .phi. of the fundamental wave.
This implies a problem that an amplifying apparatus exhibiting a full efficiency is not always realized, even if the phase deviation amount of the fundamental wave is reduced. In other words, a matching circuit has been connected which has an impedance for setting the phase deviation amount of the fundamental wave to a minimum.